Nonmetallic-sheathed cable is an art recognized designation for a power cable that is used for 600 volt service in buildings and, in accordance with the Electrical Codes of many localities, can be installed without conduit. Because conduit may be omitted, the cable must meet certain established requirements with reference to its resistance to crushing loads. It has been established that flat cables are more susceptible to crushing damage than round cables so that it is Underwriters' Laboratories practice to require crushing performance tests on flat nonmetallic-sheathed cables in the knowledge that insulation and sheathing materials and thicknesses which prove satisfactory for flat nonmetallic-sheathed cables will also be satisfactory for round nonmetallic-sheathed cables.
Underwriters' Laboratories edgewise crushing test for nonmetallic-sheathed cables require that a length of cable which includes a 180.degree. twist be compressed between steel plates 2 inches long until the insulated conductors of the cable short circuit to each other or to one of the plates. Because of the 180.degree. twist the effective crushing force is endwise. Underwriters' Laboratories flatwise crushing test for nonmetallic-sheathed cable compresses the flat surface of the cable against a 1/8 inch diameter rod. Two-conductor nonmetallic-sheathed cable, with or without a ground wire is conventionally made in a flat construction, with the conductors laid parallel and not twisted together, while three and four conductor nonmetallic-sheathed cable, which also may or may not include a ground wire, is conventionally made in a round construction. Conventional nonmetallic-sheathed cable, of which many millions of feet are in service, has solid polyvinyl chloride insulation on the conductors with minimum thicknesses increasing with conductor diameters as follows: Nos. 14-10 Awg (American Wire Gage), 30 mils; No. 8 Awg 45 mils; Nos. 6-2 Awg, 60 mils. Conventional extruded sheaths of nonmetallic-sheathed cable have also utilized polyvinyl chloride with an average wall thickness minima of thirty mils.
In U.S. Pat. No. 3,013,109 there was described a non-metallic-sheathed cable having conductors insulated with solid semirigid polyvinyl chloride which were embedded in a solid matrix of polyvinyl chloride expanded to have a gas content of 25 to 50%. The expanded matrix not only served as a sheath but entered between the conductors and filled all the available space. Largely because the conductors were protected by the dense, semirigid insulation walls this cable construction was able to meet Underwriters' Laboratories qualifications tests. Expanded insulation, as distinguished from expanded jacketing material, has been widely used for telephone conductors which operate at low voltages and are not subject to severe crushing. In telephone pair insulation the expanded compositions have the advantage over solid compositions of lower dielectric constants. British Pat. No. 742,760, published in 1956, illustrates a method of applying such expanded compositions to wire. Expanded insulation has also been widely used for video cable insulation, as disclosed in U.S. Pat. No. 2,805,276. In order to reduce the dielectric constant as much as possible, the gas content of such communication cable insulations is high, generally exceeding 25%.